A known image forming apparatus includes a sheet supply tray configured to receive a stack of sheets, which are to be fed to an image formation unit. The sheet supply tray is provided with a sheet positioning member to position the sheets not to move in the sheet supply tray. The sheet positioning member is movable relative to a bottom plate of the sheet supply tray to cope with various sized sheets.
The image forming apparatus includes a plurality of switches and a detection plate configured to move in response to the position of the sheet positioning member. The sheet supply tray is configured to determine (or detect) the position of the sheet positioning member, that is, the size of a sheet by changing the on-off states of the switches by the detection plate.